1. Field of the Invention
The invention relates to an automatic transmission control device in which gears of the transmission are shifted or changed by a plurality of friction engaging elements.
2. Description of the Related Art
Shift control systems of an automatic transmission are operated, on the one hand, by a combination of one-way clutches, multiple friction engaging elements (clutches, brakes) and band brakes and, the other hand, by a combination of multiple friction engaging elements (clutches, brakes). The shift control system includes a timing control for timing the intake and exhaust of the hydraulic operating oil supply from an operating means of a friction engaging element. The timing control can operate to exhaust the hydraulic operating oil when the oil pressure on the intake side of the operator of the friction element reaches a predetermined pressure value. This type of a timing control is operated by, for example, a shift timing valve shown in a manual for the Toyota Landcruiser New Automobile, lines 10-36, and FIG. 6 (published in 1984). The timing valve therein is a 2.fwdarw.3 shift timing valve which controls timing for draining the oil pressure of the brake B1 when the system is shifted from the 2nd gear to the 3rd gear so as to disengage the brake B1 and engage the clutch C3. The timing valve reduces the shift-shock which occurs during the shift between gear ratios. In the 2.fwdarw.3 shift timing valve, a spool 1 is urged to the upper position by the spring 2 while the vehicle is operated in the second gear. When changing to the third gear, the engaging oil pressure P.sub.c2 of the rear clutch C.sub.2 is transmitted through an orifice (not shown) from the 2.fwdarw.3 shift valve via passage L.sub.1. The spool 1 is then urged to the lower position against the oil pressure P.sub.ACC transmitted from the accumulator control valve through the passage L.sub.2 and against the force of the spring 2. When the spool 1 is moving to the lower position and the oil pressure PC.sub.2 does not increase sufficiently, the oil pressure P.sub.B1 of the brake B1 transmitted through the passage L.sub.3 only passes through the passage L.sub.4 and the orifice 3. Therefore, the engagement of the second brake B1 is maintained. When the engaging oil pressure PC.sub.2 of the clutch C.sub.2 increases further and the spool 1 moves further to a lower position, the passage L.sub.5 provided below the orifice 3 is opened and the oil pressure P.sub.B1 is quickly reduced and the engagement of the brake B1 is released. At this time, the oil pressure of the rear clutch C2 is changed to a pressure, the range of which overlaps slightly with the pressure range of the second brake B1, and the rear clutch C.sub.2 is engaged. Thereby, the shifting to the third gear ratio is completed.
FIG. 8 shows a characteristic of the C.sub.2 clutch oil pressure Pc2 and the B.sub.1 brake oil pressure P.sub.B1 at the 2.fwdarw.3 shift by the 2.fwdarw.3 shift timing valve of FIG. 7. When the solenoid of the 2.fwdarw.3 shift valve is operated by a signal to change the gear of the 2.fwdarw.3 shift at the position X, the C.sub.2 clutch oil pressure Pc2 rises to the predetermined value (a piston stroke oil pressure P.sub.B.) When the piston stroke of the C.sub.2 clutch piston has advanced, the C.sub.2 oil pressure Pc2 rises is further increased. Until the C.sub.2 clutch oil pressure Pc2 reaches the predetermined value P.sub.A (namely in the P.sub.B1 orifice drain time), as mentioned above, the B.sub.1 oil pressure P.sub.B1 is slowly reduced from the passage L4 through the orifice 3 of the 2.fwdarw.3 shift timing valve. When the C.sub.2 oil pressure Pc2 has reached the predetermined value P.sub.A (namely the value to be proportioned by the force of the spring 1 of the 2.fwdarw.3 shift timing valve and the oil pressure P.sub.ACC of the accumulator control valve being supplied from the passage L1), the spool 1 is urged to the lower position, the passage L5 is opened, and the B.sub.1 oil pressure P.sub.B1 is quickly reduced in the drain time. FIG. 9 shows the characteristic of the output shaft torque which causes the shift shock in this shifting operation. The Japanese Patent Publication No. 4940585 also shows the above mentioned structure.
In the above mentioned gear change using both multiple engaging elements, the shift shock depends on the predetermined pressure value (P.sub.A in FIG. 8) to change the shift timing value for quick drain of the engaging oil pressure of the disengaging side of the multiple element friction engaging device. The lower the shift shock the better for the automobile.
An oil circuit has a resistance to the passage of the oil flow, therefore there is a pressure difference P between the clutch oil pressure P.sub.B in the chamber of the clutch C.sub.2 and the piston stroke oil pressure PB supplied to the 2.fwdarw.3 shift timing valve (shown in FIG. 10) and, the lower the oil temperature is, the larger the pressure difference P. The oil pressure P.sub.A at the changing point of the shift timing valve is a predetermined higher value than the piston stroke oil pressure P.sub.B of the multiple engaging element of the engaging side of the engaging device. The oil pressure P.sub.A is higher than the oil pressure P.sub.B at a low oil temperature. Namely, the viscosity of the oil decreases with the increase of the oil temperature, the resistance of the oil in the oil circuit passages reduces and the pressure difference between the oil pressure P.sub.A and the oil pressure P.sub.B at the low oil temperature and the oil pressure P.sub.B at the high oil temperature increases, therefore the shift shock in the normal driving mode becomes worse.